Generally, in the process of electrostatographic printing, an insulating member, for example a photoconductor, is charged to a substantially uniform potential to sensitize the surface thereof. The charged surface of the photoconductive insulating member is thereafter exposed to a light image of an original document to be reproduced. This records an electrostatic latent image on the photoconductive member corresponding to the information areas contained within the original document. Alternatively, in a printing application, the electrostatic latent image may be created electronically by exposure of the charged photoconductive layer by an electronically controlled laser beam emanating from a raster output scanner. After recording the electrostatic latent image on the photoconductive member, the latent image is developed by bringing a developer material charged to an opposite polarity into contact therewith. In such processes, the developer material may comprise a mixture of carrier particles and toner particles or toner particles alone. Toner parities are attracted to the electrostatic latent image to form a toner powder image which is subsequently transferred to a copy sheet or equivalent media and thereafter permanently affixed by fusing.
In reproduction machines using a drum or an endless belt type photosensitive surface, the surface can contain more than one image at one time as it moves through various processing stations. The portions of the photosensitive surface containing the projected images, referred to as image areas, are usually separated by a portion of the photosensitive surface called the interdocument space. After charging of the photosensitive surface to a suitable charge level by a scorotron, the interdocument space of the photosensitive surface is generally discharged by a suitable lamp or raster scanner to avoid attracting toner particles at the development stations.
In order to accurately monitor the charge placed upon the photosensitive surface at various times, it is necessary to employ an electrostatic voltmeter (ESV) to sense the charge potential thereon. Two types of ESVs are known; contacting and non-contacting, reflecting the manner in which a probe of the ESV senses the surface charge potential. Generally, such devices include a probe or sensor working in conjunction with an associated voltmeter assembly which receives the signals from the probe and produces an output signal. Subsequently, the output signal may be used to drive an indicator, or to control an electrostatic process as a function of the measured electrostatic potential, for example, the non-contacting electrometer amplifier system disclosed by Seachman in U.S. Pat. No. 3,667,036 (issued May 30, 1972). Thus, the features of the present invention may be used in the printing arts and, more particularly, in an electrostatographic system to control a xerographic process. These electrostatic voltmeters, or ESVs, are particularly well suited for measuring photoreceptor surface charge, which in turn allows for the automated adjustment of machine characteristics to achieve high quality printed output. A problem with contacting ESVs, however, is that in order to sense the surface potential, they must physically contact the surface with the probe, for example a conductive brush as disclosed in U.S. Pat. No. 5,270,660 to Werner, Jr. et al. Unfortunately, such contacting-type ESVs typically interfere with any image deposited on the surface and are limited to use prior to development.
Heretofore, it has been recognized that in a contacting-type ESV, the sensing probe or electrode must contact the surface associated with the the field being measured in order to accurately measure the field.
Some contacting-type ESVs are described in the following issued patents which may be relevant:
U.S. Pat. No. 4,804,922 to Sometani et al. describes a voltage sensor used to sense voltage on an electrically charged object such as a transmission line. Although directed to a non-contacting sensor, Sometani et al. disclose contact type potential transformers that are affixed directly to the electrically charged object being measured.
U.S. Pat. No. 5,270,660 to Werner, Jr. et al., issued Dec. 14, 1993 and previously incorporated by reference, discloses a contacting-type electrostatic voltmeter wherein the charged surface is contacted by a conductive member. The conductive member is described as consisting of a pultruded brush material including carbon fibers therein, which acts as a suitable path to carry charge to the electrostatic voltmeter.
The present invention is a device capable of selectively actuating a probe of a contacting-type electrostatic voltmeter (ESV) wherein the measurement of electrostatic potentials is accomplished with insignificant current flow into the input of the ESV. Moreover the contact of the ESV is selectively actuated to occur only within the interdocument space, so as not to deplete the surface charge. The present invention overcomes the drawbacks of conventional contacting ESVs by allowing for the selective control of probe contact with the charged surface.
In accordance with the present invention, there is provided an apparatus for sensing an electrostatic potential on a surface and generating a signal representative thereof for input to an electrostatic voltmeter, comprising: an electroconductive brush formed of a plurality of conductive fibers; a brush support member adapted to support a first end of the conductive brush fibers; an electrical conductor, connected between the first end of the conductive brush fibers and the electrostatic voltmeter; and actuating means, associated with said brush support member, adapted to selectively move said brush into and out of contact with the surface.
In accordance with another aspect of the present invention, there is provided an electrostatic voltmeter for measuring an electrostatic potential on a surface, comprising: a surface contacting sensor for producing an output signal representative of the electrostatic potential on the surface, including an electroconductive brush, and actuating means associated with said brush, wherein the actuating means is adapted to selectively move said brush into and out of contact with the surface; a high-voltage source adapted to produce a first potential having a polarity the same as the electrostatic potential and a magnitude greater than that of the electrostatic potential; level shifting means, powered by said high-voltage source, for shifting the level of the output signal produced by said sensing means, so as to reference the output signal to a ground potential, and generating a first signal in response to the ground potential referenced output signal; and means, connected to receive the first signal from said level shifting means, for converting the first signal to a low voltage signal, wherein the low voltage signal is referenced to ground potential and where the magnitude of the deviation of the low voltage signal from ground potential is a measure of the magnitude of the electrostatic potential.